The invention provides sensor circuitry for a transducer having dual magnetic coils. The invention is particularly useful in connection with the oil pressure sensor shown in co-pending application Ser. No. 830,769, filed Feb. 19, 1986, incorporated herein by reference, though the invention is not limited thereto.
A transducer has a sensor or delta coil for generating a magnetic flux path through a path of movement of a magnetically permeable member, such as a piston which moves in response to pressure fluctuation as in the above noted co-pending application. Movement of the member changes the returned coupled flux linkage, which flux change indicates movement of the member. A reference coil is spaced sufficiently from the path of movement of the member, for example on the opposite side of the delta coil from the piston as in the above noted co-pending application, such that movement of the member has less affect on return coupled flux linkage of the reference coil than the delta coil. The reference coil compensates for environmental and ambient parameters such as temperature since such parameters affect both coils equally.
The present invention provides simple and reliable sensing circuitry for the coils. The circuit provides accurate sensing of differential voltages across the coils and provides an output error signal, which in conjunction with the above application indicates a low differential pressure or failure condition of the pump and turns off the pump motor. In the preferred embodiment, a half-wave rectified AC signal is applied to both coils in parallel and there is no relative phase shift between the coils. A differential amplifier compares the voltages across the coils and generates an amplified difference output which is compared against a dynamic floating reference provided by the half-wave rectified AC signal and which in turn provides an output error signal which charges an RC timing circuit for comparison against such error signal in a further differential amplifier whose output triggers a latching relay to turn off the pump motor.
The circuit includes various protective features preventing false tripping of the relay and preventing false resetting of the relay. The circuitry includes a safety threshold circuit supplying regulated DC as a reference regardless of the condition of the output error signal, to further prevent false tripping. The circuitry includes a subcircuit for indicating improper field installation including non-connection of the sensor circuitry to the coils or defective cable connections or the like, by ensuring that the voltage across the reference coil will be less than the voltage across the delta coil. The circuitry includes a test contact subcircuit to prevent differential voltages from being developed across the coils and prevent an output error signal, whereby to simulate a non-error condition of the pump.